U.S. patent number 6,865,320 [Application Number 10/373,613] was granted by the patent office on 2005-03-08 for optical taps formed using fiber gratings.
This patent grant is currently assigned to Fitel U.S.A. Corp.. Invention is credited to Paul Westbrook.
United States Patent |
6,865,320 |
Westbrook |
March 8, 2005 |
Optical taps formed using fiber gratings
Abstract
A matched pair of tilted gratings may be formed in a section of
optical fiber (or waveguide) and used as a "tap" to measure the
power of an optical signal passing through the fiber. By using a
pair of highly-tilted gratings (e.g., tilted at an angle of
45.degree. with respect to the optical axis) and orienting the
gratings to be orthogonal with respect to one another (i.e.,
azimuthal orthogonality around the fiber axis), a pair of
orthogonally polarized beams will be out-coupled from the
propagating signal. Since the pair of beams are orthogonal, their
sum can be made insensitive to variations in polarization of the
optical signal propagating along the fiber.
Inventors: |
Westbrook; Paul (Chatham,
NJ) |
Assignee: |
Fitel U.S.A. Corp. (Norcross,
GA)
|
Family
ID: |
34221105 |
Appl.
No.: |
10/373,613 |
Filed: |
February 25, 2003 |
Current U.S.
Class: |
385/37;
356/364 |
Current CPC
Class: |
G01J
1/04 (20130101); G01J 1/0407 (20130101); G01J
1/0422 (20130101); G01J 1/0425 (20130101); G02B
6/4215 (20130101); G02B 6/02085 (20130101); G02B
6/02109 (20130101); G02B 6/2852 (20130101); G01J
1/4257 (20130101); G01J 1/0209 (20130101); G02B
6/29323 (20130101) |
Current International
Class: |
G01J
1/42 (20060101); G01J 1/04 (20060101); G02B
6/28 (20060101); G02B 6/02 (20060101); G02B
6/34 (20060101); G02B 006/34 () |
Field of
Search: |
;385/11,37
;356/364,367 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kang; Juliana K.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application
No. 60/364,875, filed Mar. 15, 2002.
Claims
What is claimed is:
1. A polarization insensitive optical tap formed in a section of
optical waveguide including a core region for supporting the
propagation of an optical signal, said polarization-insensitive
optical tap comprising: a first highly-tilted grating disposed
along a predetermined portion of the core region of the section of
optical waveguide, said first highly-tilted grating oriented to
out-couple a first optical signal from a propagating optical signal
at a predetermined angle with respect to the optical axis of the
optical waveguide; a second highly-tilted grating disposed in
association with said first highly-tilted grating along said
predetermined portion of said core region of said section of
optical waveguide, said second grating tilted at essentially the
same angle as said first highly-tilted grating and oriented to
out-couple a second optical signal from said propagating optical
signal at an angle orthogonal to said predetermined angle, said
second optical signal exhibiting a polarization state orthogonal to
the polarization state of the first optical signal; and an optical
detector disposed outside of the section of optical waveguide at a
location to capture the first and second optical signals from the
first and second highly-tilted gratings, said optical detector for
summing the pair of output signals to remove polarization
dependence and provide an optical monitoring signal output.
2. A polarization-insensitive tap as defined in claim 1 wherein the
tap further comprising an index-matching material disposed between
the section of optical waveguide and the detector to improve the
coupling efficiency into the optical detector.
3. A polarization-insensitive optical tap as defined in claim 1
wherein the tap further comprises lenses disposed between the
section of optical waveguide and the optical detector for improving
the coupling efficiency into said optical detector.
4. A polarization-insensitive optical tap as defined in claim 1
wherein the tap further comprises mirrors disposed between the
section of optical waveguide and the optical detector for improving
the coupling efficiency into said optical detector.
5. A polarization-insensitive optical tap as defined in claim 1
wherein the tap further comprises a combination of lenses and
mirrors disposed between the section of optical waveguide and the
optical detector for improving the coupling efficiency into said
optical detector.
6. A polarization-insensitive optical tap as defined in claim 1
wherein the optical waveguide comprises an optical fiber and the
first and second highly-tilted gratings comprise a pair of gratings
formed in a core region of the optical fiber.
7. A polarization-insensitive optical tap as defined in claim 6
wherein the optical fiber comprises a birefringent optical fiber
and the pair of gratings are aligned with the two axes of
birefringence.
8. A polarization-insensitive optical tap as defined in claim 1
wherein each grating is tilted at an angle of approximately
45.degree. with respect to the optical axis of the optical
waveguide.
9. An optical arrangement comprising an all-fiber in-line
polarimeter comprising a plurality of separate gratings and an
optical waveguide disposed along a section of optical fiber, each
grating oriented at a separate, predetermined angle with respect to
the optical axis and functioning to out-couple light from an
optical signal propagating along said section of optical fiber; and
a plurality of optical signal detectors disposed alongside the
section of optical fiber and positioned so as to capture
out-coupled light from the plurality of separate gratings, the
outputs from the plurality of optical signal detectors used to
define the state of polarization of the optical signal propagating
along said section of optical fiber; and a polarization insensitive
optical tap disposed at the output of the all-fiber in-line
polarimeter to measure the output power of the propagating optical
signal, the polarization insensitive optical tap comprising a first
highly-tilted grating disposed along a predetermined portion of the
core region of said section of optical fiber, said first
highly-tilted grating oriented to out-couple a first optical signal
from a propagating optical signal at a predetermined angle with
respect to the optical axis of the optical fiber; a second
highly-tilted grating disposed in association with said first
highly-tilted grating along said predetermined portion of said core
region of said section of optical fiber, said second grating tilted
at essentially the same angle as said first highly-tilted grating
and oriented to out-couple a second optical signal from said
propagating optical signal at an angle orthogonal to said
predetermined angle, said second optical signal exhibiting a
polarization state orthogonal to the polarization state fo the
first optical signal.
10. An optical arrangement as defined in claim 9 wherein the first
highly-tilted grating of the polarization insensitive optical tap
is also used as the final grating in the plurality of gratings of
the all-fiber in-line polarimeter.
Description
TECHNICAL FIELD
The present invention relates to an optical tap and, more
particularly, to a fiber-based tap formed by using a matched pair
of fiber gratings.
BACKGROUND OF THE INVENTION
There are already known various constructions of optical
waveguides, including optical fibers, that are provided with
embedded gratings which are used for removing light at various
locations along the extent of the waveguide (i.e., an "optical
tap"). In particular, it is possible to convert a guided mode wave
in an optical fiber into a leaky mode exiting the waveguide by
forming a grating of appropriate periodicity in at least the core
region of the waveguide or fiber, thus directing the radiation out
of the optical axis. In many optical systems, an optical tap is
useful in capturing and monitoring the signal passing through the
optical waveguide. U.S. Pat. No. 5,061,032, issued to G. Meltz et
al. on Oct. 29, 1991, discloses a particular optical tap
arrangement that utilizes a blazed, chirped refractive index
grating selected to redirect light guided in the fiber such that it
comes to a focus at a point outside of the fiber. The patent also
discloses that the angle of the external path that results in the
constructive interference is peculiar to the respective central
wavelength (.lambda.).
The tap of the Meltz et al. patent exhibits shortcomings in terms
of, for example, the relatively large (e.g., greater than
22.degree.) blaze angle that is required to achieve the desired
redirection of the light guided in the fiber core to light in space
outside of the fiber, the arrangement is subject to undesirable
polarization effects, i.e., the fraction of light that is
redirected by the grating depends on the polarization of the
incident guided light. Whereas for low blaze angles
(<10.degree.) the polarization-dependent difference in the
amount of redirected light is at most about 0.54 dB, this
difference increases rapidly with increasing blaze angle, being
about 2.86 dB and about 6.02 dB for blaze angles of 22.degree. and
33.degree., respectively.
One prior art attempt to overcome the polarization-dependent
problems associated with large blaze angles is disclosed in U.S.
Pat. No. 5,832,156 issued to T. A. Strasser et al. on Nov. 3, 1998.
In the Strasser et al. optical tap, the grating is selected such
that guided mode light of a predetermined wavelength will be
directed into one or more cladding modes of the waveguide. The tap
also includes coupling means that are in optical co-operation with
the waveguide such that the cladding mode is a radiation mode. The
presence of the coupling means changes the waveguide properties in
the vicinity of the grating such that the grating directs the light
into a radiation mode (or modes). This is typically accomplished by
elimination of some or all of the cladding modes in the region of
the index grating by some appropriate type of physical means.
However, polarization-dependent problems remain with the Strasser
et al. tap, as well as other optical tap arrangements, where the
ability to out-couple a portion of a propagating optical signal
often depends upon (and changes as a result of) the polarization of
the propagating signal.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present
invention, which relates to an optical tap and, more particularly,
to a fiber-based tap formed by using a matched pair of fiber
gratings.
In accordance with the present invention, an optical tap comprises
a pair of grating structures, disposed in a contiguous relation
along the core region of an optical waveguide (e.g., fiber). The
gratings exhibit a relatively large blaze angle (e.g.,
approximately 45.degree.), where polarization-dependent problems
are overcome by disposing the pair of gratings in an orthogonal
orientation such that the out-coupled signals will be orthogonal to
one another (i.e., the scattering angle is orthogonal to the fiber
axis). The pair of signals are then directed into a pair of optical
detectors, where the output signals are then added together to form
a power monitoring output signal. By using a pair of orthogonal
out-coupled signals, any polarization-dependent variations in the
propagating optical signal are essentially eliminated.
The polarization insensitive optical tap of the present invention
is particularly well-suited for use with various other fiber-based
devices, such as an in-line all-fiber polarimeter. An optical tap
of the present invention disposed at the exit of such a polarimeter
can be used to measure the power of the output signal from the
polarimeter and modify its performance accordingly.
Various other uses and embodiments of the present invention will
become apparent during the course of the following discussion and
by reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings,
FIG. 1 illustrates an exemplary polarization-insensitive optical
tap formed in accordance with the present invention;
FIG. 2 is an end view of the arrangement of FIG. 1, particularly
illustrating the orthogonal orientation of the pair of
highly-tilted gratings;
FIG. 3 illustrates an embodiment of the present invention for use
with an array of separate optical fibers;
FIG. 4 contains a diagram of an alternative array arrangement, with
the separate fibers disposed in V-grooves in a substrate so as to
prevent the light out-coupled from one fiber to interfere with the
signal exiting any other fiber,
FIG. 5 illustrates the use of a polarization-insensitive tap of the
present invention with an in-line all-fiber polarimeter, the tap
used to measure the output power from the polarimeter;
FIG. 6 contains an alternative embodiment of the combination of a
polarimeter and an optical tap, where the final grating in the
polarimeter is used as one of the gratings in the optical tap;
and
FIG. 7 illustrates an optical tap of the present invention and an
associated shaped reflector used to redirect the out-coupled light
from the pair of fiber gratings.
DETAILED DESCRIPTION
When the period of a fiber grating is adjusted so as to couple core
guided light into light propagating approximately orthogonally
through the fiber (that is, through the cladding so as to exit
along the side of the fiber), the tilted grating will scatter the
light in a highly polarization sensitive and highly directional
manner. Importantly, the scattering is extremely broadband, since
the orthogonally scattered light has a very small interaction
length with the grating along the core region. Therefore, a single,
orthogonally-coupled grating of the present invention functions to
scatter light out of the core mode into radiation modes over a
large bandwidth and in a very directional manner, allowing for ease
of coupling into a detector placed near the fiber surface. The
period A of the grating is controlled to define the wavelength
range over which the orthogonal out-coupling will occur. The
scattering efficiency of such gratings of the present invention
will exhibit a bandwidth on the order of 50 nm over which the
scattering efficiency will remain essentially constant, with a
typical angular spread of approximately 20.degree. for the
scattered beam. Such a tilted grating thus performs the following
two functions: (1) since one polarization mode is predominantly
coupled out of the fiber, the grating functions as a highly
polarization sensitive tap that can be used to detect polarization
state within the fiber; and (2) the light transmitted through the
grating experiences polarization dependent loss (PDL), again
because the grating scatters only one polarization out of the
fiber.
The presence of polarization sensitivity in the tilted fiber
grating can therefore be used to make broadband polarization
insensitive taps if two such gratings are inscribed in the fiber.
The taps should be matched in all respects, except that they should
be oriented in orthogonal directions with respect to each other
such that they scatter orthogonal polarizations of the light
propagating in the fiber core. FIG. 1 illustrates an exemplary
polarization insensitive tap 10 which includes a first tilted
grating 12 and a second tilted grating 14 formed in a core region
16 of an optical fiber 18. In this particular embodiment of the
present invention, each grating exhibits a "tilt angle" (also
referred to in the art as a "blaze angle") on the order of
45.degree. with respect to fiber axis 20 (i.e., is "highly
tilted"). Moreover, gratings 12 and 14 are inscribed in core region
16 with their respective orientations rotated by 90.degree. with
respect to one another, resulting in forming the necessary
orthogonal coupling (see FIG. 2). In its most general case, the
tilt angle can be modified from the "highly tilted" value of
45.degree., as long as the gratings remain as a "matched"
orthogonal pair (in fact, gratings with a continuously varying
azimuthal angle and grating strength may be used). FIG. 2, an end
view of polarization tap 10 of FIG. 1, clearly illustrates the
orthogonal orientation between first grating 12 and second grating
14. With reference to FIG. 2, the azimuthal angle separating
gratings 12 and 14 about the fiber axis is maintained at 90.degree.
to achieve polarization insensitivity. It is to be understood that
although gratings in FIGS. 1 and 2 are shown as formed in different
parts of the fiber, the gratings may also be inscribed directly on
top of each other in the same section of fiber. Further, it is to
be noted that the polarization-dependent loss of the tap in
transmission will be reduced to zero in the "matched" orthogonal
condition, since equal amounts of each polarization are scattered
out of the fiber.
Referring back to FIG. 1, the out-coupled radiation A from first
grating 12 and the out-coupled radiation B from second grating 14
pass through cladding layer 22 of fiber 18 and are coupled into an
optical detector 24 located next to (or on) fiber 18. Indeed, beams
A and B from gratings 12 and 14, respectively, are coupled into a
single detector 24. A conventional pair of PIN devices can be used
in detector 24 to capture the pair of beams A and B, converting the
captured optical signals into electrical representations. In
accordance with the teachings of the present invention, as long as
tap gratings 12 and 14 are matched in strength, the light detected
by detector 24 will be insensitive to the polarization of the light
incident on the grating structure. Although gratings 12 and 14
appear next to each other in FIG. 1, they may also be overlapping
spatially within core region 16. In one embodiment, such a
structure may be formed in a highly birefringent fiber, with the
gratings aligned with one axis of the fiber. Typical grating length
for an arrangement as illustrated in FIG. 1 is approximately 1 mm,
this allowing for a few percent (e.g., less than 10%) of the
optical signal to be out-coupled by the grating.
An improvement to the optical tap of the present invention can be
made by adding an index-matching material on the outside of the
fiber. FIGS. 1 and 2 illustrate the inclusions of such an index
matching layer 26 (for example, commercially available epoxy that
is index-matched to the silica fiber) on at least a portion of the
outer surface 28 of optical fiber 18 (in a preferred embodiment,
the epoxy is disposed everywhere between the fiber surface and the
detector). A fiber grating formed in bare fiber typically exhibits
resonances in its transmission spectrum, due to resonances
resulting from reflections at the air-fiber interface. These
resonant structures result in imparting a wavelength-dependence to
the light out-coupled by the tap. By adding an index-mating
material, such wavelength dependence is greatly reduced.
The use of a matched pair of tilted gratings as polarization
insensitive optical taps is especially applicable in fiber array
arrangements, where space is at a premium. FIG. 3 illustrates a
portion of an exemplary array embodiment of the present invention,
showing (in an end view) four separate fibers 30, 32, 34 and 36,
each including a pair of highly tilted, matched gratings to form a
polarization insensitive optical tap, as illustrated in FIGS. 1 and
2, above. That is, fiber 30 includes a pair of gratings (not shown)
written in core region 40 to generate a pair of out-coupled,
orthogonal beams A and B, the pair of beams applied as an input to
a detector 50. Similarly, fiber 32 includes a pair of gratings
formed in core region 42 to tap out a pair of orthogonal beams A
and B that are directed into detector 52, and so on, for fibers 34
and 36. In order to prevent "cross talk" between fibers 30-36, some
means of preventing one light signal from entering an adjacent
detector is needed. FIG. 4 illustrates one embodiment wherein
fibers 30, 32, 34, and 36 are disposed in associated V-grooves 60,
62, 64 and 66 formed in a top surface 68 of a supporting substrate
70. Accordingly, the V-grooves are formed deep enough to block
essentially all light out-coupled by one tap (such as from core 40)
from entering the detector (such as detector 52) associated with
another tap (such as from core 42).
FIG. 5 illustrates an embodiment of the invention where a
polarization insensitive tap 80 is formed in the same section of
optical fiber 82 as an in-line, all-fiber polarimeter 84. As is
discussed in U.S. Pat. No. 6,211,957, an in-line, all-fiber
polarimeter comprises a plurality of separate gratings and an
associated waveplate to completely determine the associated Stokes
parameters and completely define the state of polarization (SOP) of
an optical signal passing therethrough. A polarization insensitive
tap 80 of the present invention, disposed at the output of
polarimeter 84 can then be used to measure the power associated
with the optical signal. FIG. 6 illustrates an alternative
embodiment of this arrangement, where a final grating 90 of
polarimeter 84 is utilized as a first grating of a polarization
insensitive optical tap 100 of the present invention. A second
grating (also tilted) is then oriented to be orthogonally coupled
(i.e., "matched") with final grating 90 so as to form a "hybrid"
in-line all-fiber polarimeter and polarization insensitive optical
tap, in accordance with the present invention. The polarization
insensitive tap may also be combined with one or two gratings that
measure only a portion of the polarization information contained in
the optical signal.
In accordance with the present invention, the taps may be formed in
birefringent fiber, and disposed so as to align with the axes of
birefringence. In this case, the taps function to measure the
amount of light in each of the two polarization modes of the
birefringent fiber.
Coupling optics of various types may be used to direct the light
from the grating to the detectors. FIG. 7 illustrates a particular
embodiment of the present invention where the out-coupled beams A
and B from a pair of orthogonal gratings 120 (shown in an end view
in this particular drawing) within an optical fiber 130 are
directed toward a light collection reflector 140 positioned
alongside of fiber 130. In this arrangement reflector 140 comprises
an elliptically-shaped mirror surface (although other geometries,
such as parabolic, may be used), with the optical axis of gratings
120 positioned at a first focal point F.sub.1 of elliptical
reflector 140. An associated detector 150 may therefore be
positioned at the second focal point F.sub.2 of reflector 140,
where beams A and B will be redirected by reflector 140 so as to
focus onto detector 150.
In the case where the gratings are not completely matched, they
will provide information on the state of polarization of the light.
Such a measurement can be included with the information from the
polarimeter to improve the polarimeter accuracy. In addition, the
fiber may be terminated with a separate detector (as opposed to an
in-line device) that measures the total power transmitted through
the grating. This detector power may be combined with the
information from the grating taps, as well as the polarimeter, to
provide polarization information about the optical signal.
While the present invention has been illustrated and described as
embodied in particular constructions, it will be appreciated that
the present invention is not so limited. For example, a
polarization-insensitive optical tap of the present invention may
be formed in a substrate-based optical waveguide instead of an
optical fiber, as discussed above. Other arrangements, including
hybrid arrangements of fibers and waveguides, are possible as well.
Thus, the scope of protection of the present invention is to be
determined solely from the claims appended hereto.
* * * * *